VARIATIONS IN ACTIVE STREAM LENGTH WITH DISCHARGE IN TWO HEADWATER STREAMS: IMPLICATIONS FOR SPATIAL AND TEMPORAL VARIABILITY IN RUNOFF GENERATION

Recent studies have shown temporary stream channels respond in complex ways to precipitation. By investigating how stream networks expand and recede throughout rain events, we may further develop our understanding of runoff generation. This study focused on mapping the expansion and contraction of two headwater catchments characterized by differing soil depths and slopes, located in North Carolina. The first watershed is a 43 ha catchment located in the Southern Appalachian region, characterized by incised, steep slopes and soils of varying thickness.The second is a 3.3 ha catchment located in the Piedmont region, characterized as low relief with deep, highly weathered soils. Over a variety of flow conditions, surveys of the stream networks were conducted at 10 m intervals to determine presence or absence of surface water. These surveys revealed several reaches within the networks that were intermittent, with perennial flow upstream and downstream. Furthermore, in some tributaries, the active stream head moved up the channel in response to precipitation and at others it remained anchored in place. The flowhead that remained stable is sourced by a seep and does not move up or down the channel. This likely indicates this stream is fed by a larger source of groundwater than the other streams within the network. This implies that there may be structural differences across the watershed. Moreover, when repeat surveys were performed during the same storm, hysteresis was observed in active stream length variations: stream length was not the same on the rising limb and falling limb of the hydrograph. These observations suggest there are different geomorphological controls or runoff generation processes occurring spatially throughout these catchments. Observations of wide spatial and temporal variability of active stream length over a variety of flow conditions suggest runoff dynamics, generation mechanisms, and contributing flowpath depths producing streamflow may be highly variable and not easily predicted from streamflow observations at a fixed point.